wil6210: allow empty WMI commands in debugfs wmi_send

There are many valid WMI commands with only header without any
additional payload. Such WMI commands could not be sent using
the debugfs wmi_send facility. Fix the code to allow sending
of such commands.

Signed-off-by: Lior David <qca_liord@qca.qualcomm.com>
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>